Scientists are excited about a new image from the second U.S. Mars rover showing the planet's underlying rock for the first time. While they plan an exploration of the site, engineers are reporting a heater problem aboard the craft.

The Opportunity rover landed several kilometers from its prime target on Mars Sunday, but mission scientists at the Jet Propulsion Laboratory in California are nevertheless jubilant.

The spacecraft rolled into a small crater 20 meters wide, where an exposed outcrop of layered bedrock has set them agog. Bedrock is a terrestrial planet's hard crust under soil and other surface materials. Mission scientist Andrew Knoll of Harvard University says the layers within it are key to understanding a planet's geological past.

"Each one of those layers records an event in the history of the planet, and by stringing them together, we develop a sense of history," he said. "So that is truly exciting, better than we could have hoped for and something that every geologist in the world is probably developing opinions about right now."

The Opportunity rover is equipped to study the rock when it rolls off its landing platform in a week or two. On its third day on Mars, the six-wheeled vehicle has transmitted a picture of the layered bedrock that has mission scientists practically salivating.

"Opportunity has now sent us the most striking image yet obtained by the Mars exploration rover mission," said lead mission scientist Steven Squyres of Cornell University, who displayed the rover pictures to reporters.

"There, look at that wonderful layer cake structure there," he went on to say. "It's going to be fascinating beyond words to get up close and personal with this thing. These are something we've never seen on Mars before. So we are about to embark on what is arguably going to be the coolest geologic field trip in human history."

The researchers want to know if the bedrock was formed from pressure on volcanic ash over billions of years or if it is sedimentary rock, which forms from sediments blown by wind or deposited by water.

Evidence of water would overjoy scientists because Opportunity and its twin rover Spirit on the other side of Mars are seeking signs that it once flowed. This would hint that the red planet might have supported life.

"If it is volcanic, then all bets are off with regard to liquid water," said Andrew Knoll. "You simply wouldn't need liquid water to form the layering in that case. If it's sedimentary, I think you need liquid water. I doubt that these are wind-born deposits."

As engineers prepare Opportunity to roll, they have found a technical malfunction. A heater that keeps the robot's instrument arm warm during science operations turns on by itself when Martian temperatures drop and is draining power from the batteries. According to mission manager James Erickson, engineers cannot control its thermostat and are evaluating whether this is going to hurt operations.

"We don't normally always want it on because we aren't normally always operating the arm. Right now, we're believing that it's going to be continuously on wherever it's cold enough," he said.

In the meantime, engineers for the Spirit rover are continuing to flush its memory of excess files that they believe caused its computer to lock up.